Jan Kleinert

Study of die break strength and heat-affected zone for laser processing of thin silicon wafers

As semiconductor based devices are manufactured on ever thinner silicon substrates, the required associated die break strength has to increase commensurately to maintain pick yields. In this study, the influence of laser processing parameters on the die break strength in laser dicing of silicon oxide-coated silicon wafers and silicon-based memory devices is investigated experimentally using ultraviolet lasers spanning a wide range of pulse width, from 400 fs to 150 ns. It is found that the net fluence, an accumulated pulse energy per surface area, is a meaningful process metric for damage induced by heat-affect zone to compare lasers processes with a large variety of pulse widths, laser scan speed, average powers, and repetition rates. Optimized process conditions for both nanosecond and femtosecond pulse widths are identified for achieving the highest die break strength in the target devices. The dependence of heat-affected zone on pulse width and net fluence during nanosecond laser processing is further...

Ultrafast laser ablation of copper with ~GHz bursts

The recently reported copper ablation study using ultrafast IR lasers with unusually high burst repetition rates (∼ GHz) that claims “an order of magnitude” efficiency enhancement compared to non-burst processes due to “ablation cooling” warrants further investigation both experimentally and through modeling the process. We experimentally reproduce a subset of these results, compare it to the known best non-burst pulse results, and find that within our experimentally accessible parameter range, there is indeed an up to ∼ 3.5x benefit when punching (i.e. drilling holes) with 864 MHz pulse bursts. However, this efficiency increase does not translate from punching to milling (machining an area), which we find to be less than half as efficient as an optimized non-burst process, while also delivering worse process quality. We conclude that a hydrodynamic picture is needed to understand the discrepancy between punching and milling efficiency for a ∼ GHz burst process.

Fast beam manipulation: Elliptical beam handling for Acousto-Optic Deflectors (AOD) in industrial laser processing

AODs have been implemented as the third level of compound beam positioning after previously combining galvanometer based scanner mirrors and translation stages in ESIs laser micromachining systems. The successful integration allows arbitrary local positioning within the AOD scan field with update rates of up to 1 MHz. Elliptical beam handling enables a cost effective trade-off between update rate and AOD scan field to maximize the AODs impact on a given laser processing task.